This invention relates to an air-fuel ratio detecting system for an internal combustion engine for detecting the air-fuel ratio of the air-fuel mixture supplied to the engine on the basis of the concentration of a gas component of engine exhaust gases.
An air-fuel ratio detecting system for an internal combustion engine has been proposed already for detecting the air-fuel ratio A/F of the air-fuel mixture supplied to the engine on the basis of the concentration of a gas component, for example, oxygen in engine exhaust gases. The proposed air-fuel ratio detecting system comprises an air-fuel ratio sensor including an element of a metal oxide semiconductor as its principal component to exhibit an electrical resistance value which is dependent upon the concentration of oxygen present in engine exhaust gases. This air-fuel ratio sensor is connected to a dividing resistor having a fixed resistance value, and the voltage appearing at the connection point between the sensor and the resistor is compared with a predetermined reference voltage in a comparison circuit so as to detect whether the air-fuel ratio of the air-fuel mixture supplied to the engine is larger or smaller than the stoichiometric air-fuel ratio.
However, the conventional air-fuel ratio detecting system, in which the dividing resistor of fixed resistance value is connected to the air-fuel ratio sensor, has been defective in that an overall shift of the characteristic curve of the electrical resistance value R.sub.e of the air-fuel ratio sensor due to the ambient temperature or aging tends to give rise to an undesirable reduction of the accuracy of detection of the air-fuel ratio, and an erroneous value of the air-fuel ratio will be detected in such a case.